The importance of fluid-injection experiments has widely increased in the past
decades mainly due to their link with energy production activities (e.g.
enhanced oil recovery, hydraulic fracturing/wastewater disposal, geothermal
energy production and CO2 storage). These activities are frequently
accompanied with the occurrence of induced micro-seismicity. While induced
seismicity is mainly perceived as a problem for the public, it can also be
used as a useful tool for efficient reservoir development and exploitation.
For this reason, both seismological and geomechanical techniques can be
combined to achieve a better understanding of the physical processes occurring
in geological reservoirs. In this dissertation, the main aim is to improve the
characterization of geo-reservoirs undergoing fluid injection under different
scenarios. To do this, the seismic responses to fluid injection from a CO2
injection project (Penn West Pilot Project, Alberta, Canada) as well as from a
geothermal field (The Geysers, California, USA) were analyzed and compared.
Particular attention was given to the analysis of the stress field orientation
by inverting focal mechanisms of induced seismicity, as well as investigating
its relation with fluid injection/extraction processes. Also, a more general
seismological and geomechanical (seismo-mechanical) reservoir analysis was
performed to efficiently illuminate the processes that led to the occurrence
of induced seismicity at The Geysers geothermal field. First, seismic and
leakage signals from a CO2 injection – Enhanced Oil Recovery project in
relation to a reported outflow of CO2/brine along the monitoring well were
investigated. Looking for CO2 leakage signatures is important to ensure the
safe storage of the CO2 within the target formation. Here, continuous seismic
recordings from a borehole array near the reservoir as well as
pressure/temperature data were examined. The seismological techniques applied
revealed no associated micro-seismicity down to MW > -1, but remarkably
elevated noise levels were observed in the seismic recordings framing the
outflow time. Additionally, the pressure sensors located in-and above the
reservoir reported leakage-related signals indicating the CO2 movement towards
the surface. In the next step, the use of the stress tensor inversion as a
tool for tracing the reservoir processes was explored. The understanding of
the stress tensor inversion allowed developing an updated version of the
linearized inversion scheme originally created by Hardebeck and Michael,
(2006) and Michael, (1987). The software package was successfully designed
together with corresponding extensive documentation system and it is now
freely available to all users. To test the correct functioning of the
software, it was applied to a natural seismicity catalog (North Anatolian
Fault Zone), induced seismicity data (The Geysers geothermal field) as well as
a synthetic catalog. Once the software package was ready, the stress field
orientation was analyzed in detail at The Geysers geothermal field. There, the
reservoir-wide distribution of the stress field orientation with depth was
investigated. Normal faulting stress regime was observed at reservoir depth,
while strike-slip regime appeared above and below the geothermal reservoir.
Considering the long production history of The Geysers geothermal field, the
changes in the stress regime were interpreted as an effect of the horizontal
stresses reduction at reservoir depth due to reservoir depletion. Then, a
specific cluster of seismicity at the northwestern part of the field was
selected to look for potential temporal stress field orientation changes in
response to fluid injection. For two injection cycles, the results reported
significant changes of the stress field orientation at the time of high
injection rates. These changes suggested that the stress field orientation
might act as a proxy to detect changes in the in-situ reservoir stresses in
response to fluid injection. Initially, the changes in the stress field were
attributed either to the reactivation of a different set of fractures, or to
the potential temporal opening of new tensile fractures. Lastly, the
previously observed micro-seismicity at The Geysers was extensively analyzed
to attain a comprehensive understanding of the characteristics of each
injection stage (i.e. before/during/after high injection rates). To obtain a
high quality dataset, seismicity relocation was performed and fault plane
solutions were re-calculated. Then the temporal evolution of a number of
seismological properties over different injection cycles was traced. Many of
these properties displayed changes in response to the peak injections and
suggested to be originating from the pore fluid pressure increase. It was
proposed that the changes displayed in the seismicity properties could be
related to the varying influence of two physical mechanisms inducing the
seismicity during each injection stage. More specifically, the thermal
fracturing of the reservoir may play the most important role regardless of the
injection stage, while poroelastic stress changes could be significant at the
times of peak-fluid injections. To finalize, a first estimation of the
complete stress tensor in this area was provided. This dissertation
contributes to the general understanding of the occurrence of induced
seismicity associated to fluid-injection projects. In addition, many of the
results obtained during this dissertation constitute a step forward with
respect to previous knowledge on the state of stress associated to fluid
injection projects.